The present invention relates to the field of digital imaging, and more particularly to representing an extended color gamut digital image with a hard-copy output medium having a limited color gamut.
In digital imaging systems, it is frequently desirable to produce hard-copy prints of a digital image. Such hard-copy prints will be limited to the range of colors and tones that can be produced on the particular device and output medium used to produce the image. This range of colors and tones that can be produced on the particular device and output medium is commonly referred to as the color gamut. In many cases, the range of colors that may be present in an original digital image may be significantly larger than the color gamut of the hard-copy print. As a result, information must be discarded in the process of producing the hard-copy print. For example, if the image contains a highly saturated blue sky that is more colorful than any blue that can be produced on the hard-copy print, the color of the blue sky must be clipped to fit within the color gamut of the hard-copy print. Likewise, an original image may also contain a luminance dynamic range (range of brightnesses) that is larger than that which can be produced on the hard-copy print. Therefore, the brightness values in the original image would need to be clipped, or otherwise compressed, to fit within the luminance dynamic range of the hard-copy print.
One example of particular importance is for images which originate on photographic negatives. This image capture means is capable of capturing a particularly large luminance dynamic range and color gamut. Some color negative systems can capture a scene luminance dynamic range of 8,000:1 or more. Typical hard-copy prints made from photographic negatives can only retain the information from a scene luminance dynamic range of about 100:1 or less. As a result, a large amount of information must be discarded in the process of producing the hard-copy print.
For example, consider the case where a photograph is taken of a person in a dimly lit room sitting in front of brightly lit window. The photographic negative has sufficient luminance dynamic range to record both the indoor part of the scene and the outdoor part of the scene. However, when a hard-copy print is produced, it is necessary to select which portion of the luminance dynamic range should be printed. A print can be made where the indoor part of the scene is well exposed, or alternatively, a print can be made where the outdoor part of the scene is well exposed. When photographic prints are produced, an automatic algorithm is typically used to guess at which part of the scene is most important and to adjust the exposure accordingly. However, many times the algorithm may guess incorrectly. In this case, the only recourse is to have another print made from the original photographic negative where the printing parameters are adjusted appropriately to properly expose the important parts of the scene.
However, a problem with this approach is that the original photographic negatives may be discarded or lost, or may be very difficult to find. As a result, it is not always possible to retrieve the extended color gamut information from the negative. In this case, it is generally not possible to make a significantly improved image from the hard-copy print alone since it does not retain any of the extended color gamut information beyond that which can be produced on the hard-copy print.
Another case where the limited color gamut of a hard-copy print can cause serious restrictions is for hard-copy images produced from digital images that are stored in a color space other than a color space specifically associated with the hard-copy output device. For example, many digital images may by stored in, or may originate in, a video RGB color space. FIG. 1 shows a comparison of a typical video RGB color gamut 10 and a typical hard-copy print color gamut 12. In this case, a*-b* cross-sections of the color gamuts are shown in the CIELAB space at an L* of 65. The colors that are inside the boundary are within the gamuts of the respective devices, while those that are outside the boundary cannot be reproduced, and are therefore referred to as xe2x80x9cout-of-gamutxe2x80x9d colors. It can be seen that there is a large set of color values that are within the video RGB color gamut 10, but are outside the hard-copy print color gamut 12. As a result, if an image in the video RGB color space was used to make a hard-copy print, it would not be possible to encode this color information. Therefore, if it were desired at a later time to make a hard-copy print on a different hard-copy output device that had a larger color gamut, it would be impossible to recover the information about the colors that were outside the color gamut of the first hard-copy print.
U.S. Pat. No. 5,841,885 to Neff et al. has disclosed a system and method of storing a digital record which is representative of an image on the print. This approach could be used to store a record of an extended color gamut representation of the image to address the present need. However, a serious shortcoming of the method of U.S. Pat. No. 5,841,885 is that a relatively large amount of information would need to be recorded on the print, thereby making this solution impractical in many cases for storing a full-resolution extended color gamut image.
It is an object of the present invention to overcome the limitations of the prior ail by permitting the retention of extended color gamut information using a hard-copy output medium having a limited color gamut.
This object is achieved in a method for representing an extended color gamut digital image on a hard-copy output medium having a limited color gamut comprising the steps of:
a) adjusting the color values of the extended color gamut digital image to fit within the limited color gamut of the output medium to form a limited color gamut digital image;
b) producing a limited color gamut output print from the limited color gamut digital image on the hard-copy output medium;
c) determining a residual image representing a difference between the extended color gamut digital image and the limited color gamut digital image; and
d) encoding the residual image on the output print using a digital encoding means such that the residual image and the limited color gamut output print are adapted to be used to form a reconstructed extended color gamut digital image.
The present invention has the advantage that the extended color gamut information associated with an original extended color gamut image can be retained on a hard-copy output print having a limited color gamut. This makes it possible to obtain the benefits associated with the original extended color gamut image source without requiring that it be archived separately.
The present invention has the additional advantage that the amount of digital data that must be recorded is substantially smaller than would be required to the entire extended color gamut digital image.
The present invention has the additional advantage that the extended color gamut information can be used, together with the original limited color gamut output print, to produce a reconstructed extended color-gamut digital image that can be used to produce improved output prints.
The present invention has the additional advantage that the extended color gamut information can be used, together with the original limited color gamut output print, to produce an improved image on an output device with a color gamut different than that of the original limited color gamut output print.